Flue gas desulfurization process

ABSTRACT

Flue gas is desulfurized by mixing into the combustion zone a finely divided sorbent such as calcium carbonate and, downstream of the mixing point, reactivating the sorbent by spraying the gaseous suspension with an aqueous solution of solubilizing agent such as a deliquescent compound or a strongly ionizing inorganic salt.

FIELD OF THE INVENTION

This invention is directed to an improved process for reducing thesulfur content of flue gas derived from combustion of asulfur-containing fuel, in which process first a dry sorbent is injectedinto the combustion zone and subsequently an aqueous solution containinga solubilizing agent is sprayed into the flue gas. Ash and sorbent canbe subsequently separated from the flue gas.

BACKGROUND OF THE INVENTION

Efforts to reduce sulfur emissions in the gaseous products fromcombustion of a sulfur-containing fuel have been made in varyingdirections. Some processes attempt to reduce or eliminate the sulfur inthe fuel prior to its combustion. Other processes propose the additionof compounds to the combustion zone which will in some manner change thenature of the sulfur compounds such that they may be more readilyremoved from the combustion products. And yet other processes removesulfur compounds from the gaseous combustion products by chemicalreaction.

U.S. Pat. No. 4,185,080, issued Jan. 22, 1980 to Rechmeier discloses acombustion gas desulfurization process wherein a powdered sorbent suchas limestone or dolomite is added to a combustion zone, and a portion ofthe solids collected from the flue gas is reactivated and can bereturned for injection into or downstream of the combustion zone. Anearlier to Domahidy, U.S. Pat. No. 3,320,906 issued May 23, 1967,teaches mixing limestone with the coal being fed to a boiler, with theflue gas then being passed to a wet scrubber for additional capture ofsulfur compounds by the calcined limestone.

The approach of chemical treatment to effect flue gas desulfurizationcan be further subdivided into wet scrubbing wherein a solution orsuspension of reagent both enters and leaves the flue gas contactingzone in liquid state, spray drying wherein a solution or suspension ofreagent enters the flue gas contacting zone in liquid state but is driedto produce a powdered solid leaving the contacting zone, and drytreatment wherein the treating reagent is a solid state powder bothentering and leaving the contacting zone.

Illustrative of the wet scrubber approach is U.S. Pat. No. 3,928,537,issued Dec. 23, 1975 to Saitoh et al, which discloses contacting theexhaust gas with an aqueous solution of an organic acid to form asoluble sulfite or sulfate. The sulfite or sulfate is removed, and theorganic acid regenerated, by a second step comprising reaction with acalcium compound such as an inorganic or an organic acid salt, forexample calcium hydroxide or calcium formate.

The spray dryer approach is illustrated for example by U.S. Pat. No.4,279,873, issued July 21, 1981 to Felsvang et al, which disclosesspraying a suspension of fresh slaked lime and recycled fly ash plusspent calcium compound into the hot flue gas in such a manner as toevaporate the slurry droplets to dryness; the resulting powdered solidsare removed from the flue gas by a downstream electrostatic precipitatoror bag filter.

U.S. Pat. No. 4,178,349, issued Dec. 11, 1979 to Wienert illustrates thedry treatment; it discloses mixing a dry, powdered lime-bearing materialin a reactor, and subsequently separating the solids from the treatedflue gas. Another patent, U.S. Pat. No. 4,442,079, issued Apr. 10, 1984,to Donelly et al, outlines a flue gas desulfurization process which isprimarily adapted to the spray dryer procedure just discussed, but isalso stated to be applicable to injection of dry sorbent at a point ofrelatively low flue gas temperature, with water being sprayed into thegas either upstream or downstream of the sorbent injection point.

Another "mixed" (wet and dry) flue gas desulfurization process isdescribed in U.S. Pat. No. 4,388,281, issued June 14, 1983 to Holter etal. In this patent, dry sorbent can be mixed with the coal prior tocombustion, but the main feature resides in splitting the flue gas intotwo parallel streams, with one stream being treated with fresh drysorbent, and the other being wet scrubbed with sorbent solutioncontaining e.g. piperazine.

Current thinking seems to be that no one of the above-discussedstrategies is the unique answer to the sulfur emission problem, eitherfor new installations or for retrofit on an existing installation.Rather, numerous site-specific factors such as proximity to reagentsource, space availability, and extent of sulfur removal required, mustenter into the selection at each plant.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a novel andadvantageous method of removing sulfur compounds, especially sulfurdioxide, from combustion exhaust gas.

According to the present invention, flue gas is desulfurized by firstcontacting it in the combustion zone with a finely divided dry sorbent,and then reactivating the resulting gaseous suspension of calcinedsolids by spraying into it at a particular location downstream of theheat recovery exchangers an aqueous solution of a solubilizing compound.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, the sole FIGURE represents a flow diagram, partiallyschematic, of a flue gas desulfurization process according to ourinvention.

DETAILED DESCRIPTION OF THE INVENTION

Flue gas containing sulfur dioxide from combustion of asulfur-containing fuel is typically passed from the combustion chamberthrough heat recovery exchangers and thence by way of duct work to astack for disposal. Where the fuel is a solid, such as coal, or a heavyliquid, the combustion product gases will also contain ash solids suchas fly ash, in which instance the flue gas will first be treated forsolids removal by such as a bag filter or electrostatic precipitator(ESP) prior to being passed to the stack.

The invention will now be described by referring to the drawing. Acombustor such as boiler 10 is provided with a burner 12 which isadapted to introduce a pulverized solid fuel such as coal from conduit14 and a combustion-supporting gas such as air from conduit 16. Hotcombustion products leave the upper region of boiler 10, travelling pastand through various heat exchange surfaces which typically extract heatby radiant and convective transfer, until they are discharged from theboiler assembly at duct 18. The final stage of heat recovery from theflue gas is often an air preheater, shown here as 20, which serves topreheat combustion air to conduit 16. Past practice has been that fluegas leaving air preheater 20 by way of duct 22 is passed directly tosolids removal to separate out fly ash, such as by a bag filter orelectrostatic precipitator 24, and thence discharged by a stack 26.

According to our invention, the combustion process is treated forreduction of its sulfur dioxide and sulfur trioxide content by injectionof a finely divided dry sorbent powder such as calcium carbonate,magnesium carbonate, or mixtures of these two, into boiler 10 by way ofa conduit 56. The dry powder can be carried through conduit 56 by use ofa transport gas such as air. Further according to our invention, a shortdistance downstream from preheater 20, duct 22 enters humidifier 32,wherein solubilizing solution from conduit 34 is sprayed into the fluegas. Humidified flue gas passes by way of duct 36 to solids collector 24and thence to stack 26. Solids removed in collector 24 will normallycomprise a mixture of fly ash and spent sorbent, and are discarded towaste by way of conduit 42.

Returning now to a discussion of the sorbent, it is added in itsrelatively inexpensive but relatively unreactive carbonate form, aslimestone or dolomite, at conduit 56. This material can either be addedto boiler 10 by way of conduit 58 into burner 12, or by way of conduit60 through separate injection ports into an upper region of thecombustion zone, depending upon the prevailing temperature profile andresidence time. In either event, the limestone or dolomite is calcinedby the elevated temperature. The limestone or dolomite useful in thepresent invention is finely divided, and preferably of a size range thatat least about 70 weight percent passes a 200 mesh Tyler sieve screen.In order to effect calcining, the material must reach a temperatureslightly in excess of about 1000° C. but in order to avoid deadburning,as is known in the calcination art, it should not be subjected totemperatures much in excess of 1250° C. for any appreciable time. Theresulting suspension of calcium and/or magnesium oxide is thus affordedsignificant reaction time for flue gas desulfurization in duct 18 andair preheater 20.

The process of the present invention is especially adapted to removesulfur dioxide from a flue gas wherein availability of relatively shortresidence time in the duct work upstream of the ESP (as in a retrofitinstallation) and/or a relatively high concentration of sulfur dioxidecombine to present difficult removal. The humidification of the flue gasand reactivation of its entrained sorbent powder by spraying watercontaining solubilizing agent is an essential aspect of our invention.As solubilizing agent we presently prefer sodium hydroxide, althoughother useful materials include sodium carbonate, calcium chloride,adipic acid, and glycerol. Although we do not wish to be so bound, wetheorize that compounds which form highly ionized solutions or which aredeliquescent serve to form a thin liquid film of greatly increasedreactivity toward sulfur dioxide on the surface of each of the particlesof gas-suspended solid sorbent, and thus serve to activate calcinedparticles which have not reacted. More importantly, we believe that thethin liquid film tends to bring fresh unreacted calcine to the surfaceof those particles which have already reacted, thus increasing overallsorbent utilization.

It is necessary for several reasons to control the humidity, or approachto saturation, of the flue gas containing sorbent and sprayed solution.It is preferable to approach the dew point at the inlet to the solidscollector as close as possible without causing operating problems, sincewe postulate that a more humid flue gas permits protracted existence ofa liquid film on the sorbent, and the reaction of gaseous sulfur dioxidewith liquid solution is much more rapid than with a solid particle. Onthe other hand, if the dew point is approached too closely, the spraydroplets tend to foul and plug the surfaces of bag filters or ESPcollector plates, and to cause excessive corrosion of duct work. Weprefer to approach to within at least about 35° C. of, but no nearerthan about 10° C. of, the saturation temperature.

The degree of desulfurization achieved is also controlled by the Ca/Sratio, i.e. the amount of calcium (and/or magnesium) compound in thesorbent as compared to the amount of sulfur dioxide in the flue gasbeing treated. This ratio is normally expressed in moles, and we preferthat it be in the range from about 0.5:1 to about 3:1. It is controlledby the flow rate in conduit 56. Solubility of the solubilizing salt,e.g. calcium chloride, sodium hydroxide, etc. in water determines themaximum concentration of spray solution. It is advantageous that theduct work provides a residence time for the humidified solids of atleast about 1 second, and preferably more.

In the usual application of this invention, the sulfur-containing fuelbeing burned will also contain a significant proportion of ash-formingconstituents, and thus the flue gas will contain fly ash. Addition ofsolubilizing (humidifying) solution and dry sorbent according to thepresent invention causes an increase in the solids loading of the fluegas, and thus on the duty to be served by the downstream solidscollector bag house or ESP. However, it has been determined that thesolids retained on the surfaces of such solids collector remain activefor SO₂ absorption for a longer period when humidifed with asolubilizing agent according to the present invention, particularly whenthe desired close dew point approach is maintained.

The temperature of the flue gas at the point of injection of thesolution via conduit 34 should be between about 120° and about 230°, andpreferably between about 140° and about 175° C.

The invention will now be illustrated by the following examples.

EXAMPLE I

An electrical utility station burning 2.4 weight percent sulfur coal istreated for reduction of sulfur dioxide emission according to the methodof the present invention. Into an upper region of the boiler is injectedby way of conduit 60 an air suspension of finely divided limestone inamount sufficient to provide a resultant Ca:S molar ratio of about 2.5.Next, a 2 weight percent solution of sodium hydroxide in water issprayed into the gas stream by way of conduit 34 in amount sufficient toresult in a gas temperature about 10° C. above the dew point at the ESPinlet. The flue gas velocity in the ducts 22 and 36 is such that thesorbent particles have a residence time of about 2 seconds prior topassage into the ESP. Average residence time of collected solids in theESP is about 22 minutes. The SO₂ content of the gas exit the ESP isabout 440 ppmv, dry (parts per million by volume, dry basis).

EXAMPLE II

The test of Example I is repeated, except that neither limestone sorbentnor sodium hydroxide solution is added. The SO₂ content of the gas exitthe ESP increases to about 1760 ppmv, dry.

EXAMPLE III

As a basis for comparison, Example I is repeated except that nosolubilizing solution is added through conduit 34. The sulfur dioxideanalysis exit the ESP increases to 700 ppmv, dry.

EXAMPLES IV-VI

Solutions of calcium chloride, sodium carbonate, and glycerol aresequentially substituted for the sodium hydroxide solution in Example I,with similar sulfur dioxide removal results.

EXAMPLE VII

Example I is repeated, except that the quantity of sodium hydroxidesolution is decreased to the point that the dew point approach is onlyabout 25° C. Analysis of SO₂ exit the ESP increases to about 800 ppmv,dry.

EXAMPLE VIII

Example I is repeated, except that the limestone rate in conduit 60 isdecreased to provide a Ca:S molar ratio of about 1. The SO₂ analysisexit the ESP increases to about 970 ppmv, dry.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure and the appended claims to our invention, theessence of which is that we have provided an improved method forreducing the sulfur dioxide content of flue gas by reaction with sorbentof increased activity, and for preparing such sorbent.

What is claimed is:
 1. The method for reducing sulfur dioxide content ofa flue gas resulting from combustion in a combustion zone of asulfur-containing fuel, which method comprises:(a) injecting into saidcombustion zone a finely divided dry sorbent comprising calciumcarbonate in amount sufficient to provide a metal salt:sulfur ratio ofat least about 0.5:1; (b) spraying into the resulting suspension ofsorbent in flue gas at a point where said flue gas has a temperature ofbetween about 120° and about 230° C. an aqueous solution of solubilizingagent, such agent being selected from sodium hydroxide, sodiumcarbonate, calcium chloride, adipic acid and glycerol; (c) providing acontact time between said flue gas and droplets resulting from saidspraying of at least about 1 second; (d) subsequently separating fromsaid flue gas solids resulting from drying of said droplets and solidsresulting from combustion of said fuel; (e) discharging from saidseparating a flue gas of substantially diminished sulfur dioxidecontent; and (f) regulating the rate of said spraying relative to therate of said flue gas such that the temperature of said flue gas at thepoint of said separating is between about 10° C. and about 35° C. aboveits saturation temperature.
 2. The method of claim 1 wherein saidcalcium carbonate comprises limestone of a size such that at least about70 weight percent passes a 200 mesh Tyler screen.
 3. The method of claim2 wherein said injecting is by way of burner means extending into saidcombustion zone.
 4. The method of claim 2 wherein said injecting is byway of injection port means located above burner means extending intosaid combustion zone.
 5. The method of claim 1 wherein said separatingof step (d) consists of subjecting said flue gas to electrostaticprecipitation.